This invention relates to the field of thermal ink jet printhead systems, and more particularly, this invention relates to a multiple output driver circuit used for driving heat elements associated with thermal ink jet printhead systems.
Modern ink jet printing systems use a printhead having a plurality of ink flow channels that connect with an ink reservoir. Each ink flow channel terminates in a nozzle through which ink is expelled. A heating element, usually formed as a resistor, is associated with each ink flow channel. This heating element is usually positioned at the bottom portion of the ink flow channels and spaced a predetermined distance from the nozzle. A power driver circuit supplies a pulse of current to each heating element at a predetermined time, thus heating the heating element. As a result, the ink vaporizes and forms an ink bubble. As this ink bubble grows, the ink is ejected out of the nozzle. When the current from the power driver circuit is stopped, the heating element cools and the ink bubble collapses. The ink located at the vicinity of the nozzle is then pulled back into the nozzle and into the ink flow channel, resulting in an ink drop that ejects from the nozzle by separation of that portion of the ink located outside of the nozzle from that portion of the ink located inside the nozzle. Usually the power driver circuit is formed as a power output transistor (such as a power MOS transistor) that drives the heating element formed as a resistor. A logic circuit connects to the power output transistor and signals when the power output transistor should turn on to heat the respective heating element.
The energy delivered to the resistor must be controlled. Thus, it is important to measure and predict accurately the energy that is obtained when a resistor is xe2x80x9cfiredxe2x80x9d, i.e., heated, to vaporize and eject ink from the nozzle. Temperature fluctuations during the printing process often cause variations in energy. Usually, there are twenty to fifty nozzles located within a printhead, with one resistor per nozzle. Thus, the temperature fluctuations can vary by as much as 10% or 15%.
In some prior art applications, a ballast resistor was used. It was driven from a 24 volt supply and power was drawn down to the top of the nozzles. An FET transistor, a power MOS transistor, was turned on. A variation in the VDS with the power MOS transistor represented a variation in the energy. It is necessary to obtain better control of the VDS of the transistor. The energy delivered to the resistor is effected by the RDS for the xe2x80x9conxe2x80x9d position of a device, and thus, it is desirable to control the voltage drop across the transistor output.
One prior art technique is shown in FIG. 2, which illustrates a technique to sense the output voltage of each power transistor when the transistor is xe2x80x9con.xe2x80x9d That voltage is compared to a reference and the gate of the power transistor, usually formed as a power MOS transistor, is controlled so that the output voltage is regulated. However, this system has several disadvantages because it requires an amplifier for each power MOS transistor. The amplifier must have a very high bandwidth when used in a power driver application where output pulses are 2-4 microseconds wide. This technique also requires one amplifier circuit per transistor and requires much area on the semiconductor circuit die.
It is therefore an object of the present invention to provide a multiple output thermal ink jet printhead driver that does not require an amplifier circuit per each power output transistor.
It is still another object of the present invention to provide a multiple output driver circuit, such as for driving a thermal ink jet printhead, which uses only one reference circuit to regulate the RDS on a reference transistor and the RDS of a power output transistor, such as a power MOS transistor.
In accordance with the present invention, a thermal ink jet printhead has a base member. A plurality of ink flow channels are positioned in the base member and connect with an ink reservoir. The ink flow channels terminate in a nozzle through which ink is expelled. A heating element is associated with each ink flow channel. A multiple output driver circuit is connected to the thermal ink jet printhead and is formed as a semiconductor integrated circuit.
The multiple output driver circuit includes a power output transistor connected to each heating element. Each of the power output transistors includes a gate. A reference circuit is operatively connected to each gate of each power output transistor. The reference circuit includes a reference transistor having a gate and a reference amplifier that receives a reference voltage and a source of current. An amplifier output is operatively connected to the gates of the power output transistors and the gate of the reference transistor. The reference amplifier regulates the reference transistor directly and the power output transistors by matching.
In still another aspect of the present invention, a logic circuit is operatively connected to each of the gates of the power output transistors. The logic circuit can be monolithically formed with the semiconductor integrated circuit. The heating element connected to each ink flow channel comprises a resistor in one preferred aspect of the invention. The resistors can be monolithically formed with the printhead.
In still another aspect of the present invention, the plurality of power output transistors each comprise a power MOS transistor. A gate driver circuit is directly connected to each gate of each power output transistor and is typically formed as a push/pull transistor circuit. Each power output transistor can include a source terminal connected to ground and a drain terminal connected to a voltage supply.
In still another aspect of the present invention, a method of operating a thermal ink jet printhead is disclosed and claimed. The method operates a thermal ink jet printhead and comprises the steps of regulating a reference transistor of a reference circuit directly, while matching with the reference circuit a plurality of power output transistors that are connected to a resistor of a thermal ink jet printhead. The reference transistor of the reference circuit and the power output transistors can be monolithically formed as a semiconductor integrated circuit. Logic signals are input to gate drivers that are connected to each of each power output transistor to turn on respective power output transistors. The power output transistors can be formed as power MOS transistors.